Saracens suffered a second defeat in three at Bath, while Sale Sharks piled on the agony for Leicester Tigers.
The Hurricanes lead the way in Super Rugby after a 25-22 win over the Highlanders, while the Rebels triumphed 29-26 against the Brumbies.
A 28-9 victory over Stade Francais saw Top 14 leaders Toulouse break a league record as they extended their unbeaten run to 12 games.
Ulster saw out the final 10 minutes with 13 men to inflict a four-try defeat on a beleaguered Dragons side.
Newcastle Falcons closed the gap on second-bottom Worcester Warriors to five points with a 17-6 victory in the Premiership on Sunday.
A late rally away to Jaguares was not enough for the Blues to avoid suffering a third successive Super Rugby defeat.
Toulouse's lead in the Top 14 table is down to a solitary point after Clermont Auvergne thrashed Grenoble. Lyon, meanwhile, lost at home.
Saracens eased past Northampton Saints but table-topping Exeter Chiefs had to repel a second-half rally from Sale Sharks.
Handre Pollard scored 20 points with his boot as the Bulls tamed the Lions, while the Sharks tasted defeat for the first time this...
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The Sunwolves rocked the Chiefs to claim their first away Super Rugby victory and the Crusaders were too strong for the Reds.
Gloucester let Friday's Premiership match slip through their fingers late on at Bristol Bears.
A 15th Pro14 win of the season was enough for Leinster to make sure they will be at home in the semi-finals this term.
Ngani Laumape tormented the Brumbies as the Hurricanes dished out a hammering, while the Rebels made it two wins out of two.
Lyon claimed a third successive win and are breathing down Clermont's necks after beating them at Matmut Stade de Gerland,
Italy deservedly led Ireland 16-12 at half-time in Rome, but Joe Schmidt's side just about did enough to avoid a huge Six Nations upset.
Worcester Warriors eased their relegation fears with a third victory of the season over Leicester Tigers, who have lost five in a row.